Functional magnetic resonance imaging (fMRI) has become one of the most important new imaging tools in clinical neuroscience, due to its ability non-invasively to measure alterations in cerebral blood flow and neural activation in response to external stimuli or cognitive tasks without the use of radionuclides. The ability to monitor these parameters in a safe and repeatable manner with the high spatial and temporal resolution provided by FMRI permits investigators to design a wide variety of experiments to explore brain function.
fMRI methods are based on the observation that changes in local magnetic susceptibility in the brain, within an externally applied magnetic field (as in an MRI scanner), are associated with focal changes in the magnetic resonance parameters of nearby tissue. Changes in magnetic resonance parameters result in time dependent local image intensity variations. Although these susceptibility variations can be detected using conventional MRI techniques, echoplanar MRI is usually preferred for these experiments due to its significantly greater temporal resolution. By recording images at a high rate, small susceptibility changes in vivo can be observed on physiological time scales with a high degree of reliability.
Time-dependent susceptibility changes typically arise from at least two sources. The first is from endogenous changes in blood oxygenation level in response to neural activation, which affects the ratio of diamagnetic and paramagnetic forms of hemoglobin. These susceptibility changes increase image intensity in T2* weighted pulse sequences by at most 2-5% at 1.5 Tesla (Turner et al., Magn. Reson. Med. 29:277-279, 1993). Several cycles of response to a repeated stimulus can be averaged together to increase the SNR. This limits experiment design significantly, as it is unsuitable for observing transient phenomena or those that change with repeated stimulation.
The second major source of susceptibility contrast arises from the first pass of exogenously applied paramagnetic contrast agents through the cerebral vasculature (Vrillinger et al., Magn. Reson. Med. 6:164-174, 1998; Rosen et al., Magn. Reson. Q. 5:263-281, 1989). Perfusion experiments using contrast agents show image intensity changes of up to 20%. However, they cannot be repeated immediately, so data must be collected in a single pass. The single shot SNR of the echoplanar imaging process is therefore a limiting factor in the magnitude of the effects that can be discerned with fMRI.
One method that has been used to increase the SNR of fMRI has been the use of receive-only radiofrequency (RF) surface coils, which optimize the SNR in a desired region of the brain, while sacrificing image quality in other regions. While this is useful for many types of experiments, it can be difficult to design a single coil with the desired response profile, and experiments that require measurements in multiple separate brain regions cannot benefit from this technique.
In recent years, conventional MRI scanners have been significantly improved through the use of phased-array detectors. (Roemer et al., Magn. Reson. Med. 16:192-225, 1990; Hayes et al., Magn. Reson. Med. 18:30914 319, 1991; Hayes et al., Radiology 189:918-920, 1993). In the MR phased array, signals from multiple independent surface coils are combined to increase both sensitivity and spatial coverage compared to a single coil, while covering large arbitrarily shaped regions. Phased array coils for conventional neuroimaging have demonstrated up to 30% increases in SNR relative to a single surface coil, and 3-4 fold increases in spatial coverage. (Wald et al., Magn. Reson. Med. 34:440-445, 1993). In experiments where a volume head coil would normally be used to cover the entire head, such as perfusion experiments, phased array coils have shown 50-200% increases in sensitivity compared to a quadrature head coil, while still covering most of the cerebrum.
Although a number of conventional scanners are now equipped for use with phased array coils, and many systems are now adding echoplanar capability, current systems have no way to use these capabilities in tandem. Echoplanar imaging places stringent requirements not only on the gradient hardware of an MR system, but also requires a very high bandwidth RF receiver. To date, commercial MR systems have implemented phased array capability by replicating conventional RF receiver chains, and rely on a single, separate, echoplanar receiver for use in high speed imaging.
One proposed engineering solution to the problem of adding additional receiver channels to an echoplanar system is to multiplex analog data from multiple coils in either the time or frequency domain, and use a single RF receiver/digitizer to record the data, leaving the rest of the data acquisition system unmodified (except for processing software). This analog multiplexing option adds few additional parts to the MR system, so the additional hardware may be less expensive. A preliminary report describes a phased array echoplanar system being developed using this technique. (Wu et al., Time Domain Multiplexing Phased Array in EPI. in 5th Scientific Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine. Vancouver BC, Canada, 1997. In this approach the analog bandwith of the receiver becomes a data bottleneck, and interleaving the multiple data streams into a single analog channel requires a reduction in the image resolution or signal to noise ratio of the individual coils, or both. This can be alleviated through modification of the receiver, but this in turn can cause an SNR penalty when performing single coil studies. Using the same receiver for multiple RF coils can also introduce correlated noise between the individual channels, offsetting one of the major SNR benefits of phased array coils. Further, minimizing the crosstalk between the analog data channels requires that analog multiplexing system remain properly tuned over time. This requires careful design to ensure system stability. Another more subtle problem is that certain multiplexing schemes can introduce time delays between samples taken on different channels. This can lead to ghosting artifacts, requiring special processing to mitigate this effect.